1. Field of the Invention
This invention is directed to the field of transmission circuits, and more particularly, to level shifter used to transmit control inputs to drive power semiconductor modules
2. Description of the Related Art
The invention is directed to a level shifter used with a drive circuit, and an associated method for transmission of an input signal from drive logic to a driver. Drive circuits such as these are required in power-electronic systems to drive power semiconductor switches which are arranged as individual switches or in a bridge circuit. Bridge circuits such as these are known as single-phase, two-phase or three-phase bridge circuits, with the single-phase so-called half-bridge representing a basic module of a multiplicity of power-electronic circuits. Two power switches, a first so-called TOP switch and a second so-called BOT switch, are arranged connected in series in a half-bridge circuit.
In general, a half-bridge such as this is connected to form a direct-current intermediate circuit. The output, typically the alternating-voltage connection of the half-bridge, is generally connected to a load. In general, the drive circuit comprises a plurality of circuit elements or functional blocks. The control signal is preprocessed in a first circuit, known as the drive logic, and is supplied via further components to the driver circuits and finally to the control input of the respective power switch.
For relatively high intermediate-circuit voltages, for example of more than 100 V, the drive logic is generally DC-isolated from the driver circuits since the associated power switches are at different potentials, making DC isolation essential. This isolation is required at least for the TOP switch although, for relatively high power levels, it is also applicable to the BOT switch, because of the ground potential possibly being dragged during switching. By way of example, such isolation may be provided by pulse transformers, by optocouplers or optical waveguides (galvanic isolation) or with the aid of integrated circuit technology using an HVIC (High Voltage Integrated Circuit). The latter variant is used increasingly frequently because of various advantages, such as small dimensions, low price and long life. At the same time, HVICs offer the capability to integrate a high-voltage component with a breakdown voltage that is greater than or equal to the intermediate-circuit voltage which can be used in circuits for signal level conversion, in so-called level shifters. A lateral high-voltage MOSFET is generally used for this purpose.
The described level shifter is part of the drive circuit and is preferably in the form of an integrated circuit arrangement. It is used to transmit a signal from a circuit part at a first defined reference-ground potential to a circuit part at a second reference-ground potential which is higher or lower at times then the first reference ground potential, or vice-versa. An arrangement such as this is required for an integrated and DC-isolated drive for power semiconductors.
Two fundamental isolation technologies are known to form level shifters for HVICs (Silicon on Insulator) technologies on the one hand and pn-isolated technologies (Junction Isolation) on the other hand. SOI technology offers dielectric isolation of components and component groups, but is available at the moment only up to a withstand voltage of 800 V. SQI substrate wafers are considerably more expensive than standard substrates, although the costs are compensated for by a number of advantages and considerable process simplifications which result from the dielectric isolation. In the case of pn-isolated technologies, the reverse voltage is blocked by a reverse-biased ˜0 junction. This technology is available for up to 1200 V at the moment. However, production is highly complex and therefore costly. Furthermore, there are technical problems, for example with leakage currents and latch-up effects, inter alia at relatively high temperatures, for example at an operating temperature of more than 125° C., and when the ground potential is dragged during first dynamic processes.
In integrated drive circuits according to the prior art, level shifter transmission of the driver signals from the drive logic to the TOP driver is known. This is necessary since the TOP driver, in contrast to a BOT driver, is at a higher reference-ground potential, on a phase basis. According to the prior art, the signal transmission from the drive side to the TOP driver takes place by means of pulsed (dynamic) and differential transmission, that is to say switch-on and switch-off pulses are produced on the drive side from the signal to be transmitted, and are transmitted via the respective level shifter to the TOP driver. This type of transmission is distinguished by a high level of transmission reliability and low power consumption. Various integrated level shifter topologies are known. The simplest topology comprises an HV transistor with an appropriate blocking capability and a resistor, connected in series. When a signal is passed to the gate of the HV transistor, it switches on. The parallel current produced in this way through the level shifter causes a voltage drop across the resistor, and this can be detected as a signal by an evaluation circuit.
German patent application DE 101 52 930 A1 discloses an upgraded level shifter topology, in which the drive signal is transmitted in steps by means of n−1 intermediate potentials by means of n known level shifters which are connected identically in cascade. This makes it possible to use transistors which have only the n-th part of the required blocking capability of the entire level shifter. If transistors with the required blocking capability are available, the blocking capability of the level shifter can be increased by the factor n.
German patent application DE 10 2006 037 336, discloses a level shifter in the form of a series circuit formed by n series-connected HV transistors. This topology has the advantage over that disclosed in DE 101 52 930 A1 on the one hand that the power consumption is reduced and on the other hand that the circuit complexity is reduced. This results in less space being required and in reduced costs.
All the known topologies have the common feature that, with a complementary level shifter design, signals can also be transmitted from a circuit part with a high reference-ground potential to a circuit part with a low reference-ground potential. This characteristic can be used to transmit signals back from the TOP driver to the drive logic.
According to the prior art, in the case of integrated drive circuits, the drive logic (on the primary side) and the BOT driver (on the secondary side) are at the same reference-ground potential, or at reference-ground potentials which differ from one another by only a few volts, so that there is no need to transmit signals via level shifters. In this case, the connections for the reference-ground potential on the primary side and on the secondary side are generally externally shorted. However, module-internal and system-internal inductances, for example line inductances, can result in the reference-ground potential of the BOT drivers being dragged severely in the positive or negative direction during switching of the power component. This occurs particularly severely in medium-power and high-power systems in which heavy currents, for example of more than 50 A are switched. The potential difference can in this case assume values which are higher than the blocking voltage of the gate oxide of the transistors being used, for example more than 20 V. Junction isolation technologies have the disadvantage that parasitic thyristor structures can be triggered, so-called latch-up, in the negative direction if the reference-ground potential is dragged in a corresponding manner. This leads to loss of function and possibly to destruction of the components affected. SSOI technologies are not subject to this restriction, caused by the dielectric isolation of the components, so that it is possible to implement level shifter circuitry which ensures reliable signal transmission even if the reference-ground potential on the secondary side becomes negative, whether briefly or permanently.
German patent application 10 2006 050 913 discloses a level shifter such as this for BOT drivers using SOI technology in the form of an UP and DOWN level shifter path. However, this drive circuit is not adequate for a bridge topology, since the reference-ground potential of the TOP driver on the secondary side may also be more negative in the reference-ground potential on the primary side.